A variable valve controller is mounted on an internal combustion engine in order to improve output and fuel efficiency and to reduce emission of the internal combustion engine. The variable valve timing controller varies the camshaft phase so that the valve timing of the intake valve and/or exhaust valve is adjusted.
Japanese Patent No. 3290427, which is a counterpart of U.S. Pat. No. 6,213,070, shows one of methods to detect the camshaft phase. In this method, a crank angle sensor outputs crank angle signals every when the crankshaft rotates a predetermined crank angel. A cam angle sensor outputs cam angle signals every when the intake camshaft rotates a predetermined camshaft angle. Rotational speed information is calculated based on the cam angle signals every when the camshaft rotates a predetermined sampling angle. The rotational speed information is, for example, a time period T110 in which the crankshaft rotates 110° CA. Based on the rotational speed information, a time difference between output timing of the crank angle signals and output timing of the cam angle timing is converted into a rotational angle difference in order to derive the camshaft phase.
It has been required that the valve timing is precisely adjusted even when the engine is cranking or idling in an extremely low speed. In the above-described method, the sampling angle is constant without respect to the engine speed. For example, the sampling angle is 110° CA. Thus, when the engine speed is extremely low, the fluctuation of the engine speed is increased, so that the accuracy of the camshaft phase is deteriorated because the sampling angle is too large. It is hard to adjust the valve timing precisely.